The collision-induced absorption of H2 near 1.20 μm: Subatmospheric measurements and validation tests of calculations

The weak binary collision -induced absorption (CIA) of molecular hydrogen is measured at room temperature in the first overtone region near 1.20 mu m. Binary absorption coefficients are derived by cavity ring down spectroscopy (CRDS) at 28 selected spectral points sampling the (2-0) band between 7974 and 8650 cm -1. While all previous studies used high density samples, the sensitivity of the CRDS method allowed deriving accurate CIA by using pressure ramps of pure H2 limited to a maximum pressure of 1 atm. After subtraction of the Rayleigh contribution, a purely quadratic pressure dependence is obtained for the absorption coefficient at each measurement point and the CIA binary coefficients are derived with a 1.5 % accuracy. The comparison to theoretical values widely used for astronomical applications shows deviations values between 5 and 25 %.